The invention relates to a capped vehicle headlight lamp provided with a translucent lamp vessel which is sealed in a vacuum-tight manner, which is filled with an ionizable gas and in which a pair of electrodes is arranged. Current lead-through conductors extend through the wall of the lamp vessel to the pair of electrodes. The lamp cap is comprised mainly of electrically insulating material and is firmly connected to the lamp vessel. Current supply conductors have first ends connected to contacts at the lamp cap and second ends secured to a respective lead-through conductor.
Such an electrical discharge lamp is known from European Patent Application No. 0 152 649 corresponding to U.S. Ser. No. 685,709, now U.S. Pat. No. 4,634,920 use for the lamp according to the invention requires that the lamp vessel be firmly connected to the lamp cap because of shocks and vibrations to which the lamp will be subjected. The pair of electrodes must have an accurately defined position with respect to a reference point at the lamp cap in order that the discharge path between the pair of electrodes will occupy the correct position with respect to the focus of the reflector of a headlight lantern in which the lamp is arranged.
In the known lamp, these requirements are satisfied for the major part. One end of the lamp vessel is arranged with clamping fit in a metal plate having a cylindrically flanged edge. The lamp cap has a cavity in which a metal sleeve is fixed, which cooperates telescopically with the said metal plate and is secured to it after the lamp vessel has been set to the correct position. In this lamp, the use of cement is avoided. This is important because cement can absorb moisture, which may lead to corrosion of a reflector, and because cement can become brittle and can crumble off under the influence of varying thermal and mechanical loads.
Nevertheless, the known lamp has a number of disadvantages. The lamp has a large number of components, which have to be joined and secured to each other. Furthermore, in an electrical discharge lamp used as a vehicle headlight lamp in contrast to an electrical incandenscent lamp for this application, the dimensions of the end of the lamp vessel are sometimes so small that it is very difficult to ensure that the metal plate firmly holds the lamp vessel. This is the case especially if the lamp vessel has two oppositely arranged ends, where a respective current lead-through conductor is passed through the wall of the lamp vessel.
Another disadvantage is that the lamp has to burn during its alignment when it is secured to the lamp cap. Due to the above mentioned disadvantages, the operation of securing the lamp vessel to the lamp cap is difficult, time-consuming and hence expensive.
British Patent Application No. 2,132,011 corresponding to U.S. Pat. No. 4,594,529 discloses a capped electrical discharge lamp for use as a vehicle headlight lamp, in which a dripping cap extends along the lamp vessel, which during operation of the lamp intercepts a part of the emitted light in order to assist in forming a dipped light beam. In such a lamp, not only the lamp cap and the lamp vessel, but the lamp cap, the lamp vessel and the dipping cap have to be aligned with respect to each other, which makes the manufacture of the lamp even more complicated.
In the above British Patent Application, the lamp cap and the means for securing the lamp vessel thereto are indicated only diagrammatically. It is not clear how it is achieved that the lamp vessel, the lamp cap and the dipping cap are aligned with respect to each other.